Phospholipid metabolism was found to play a significant role in the in vitro recognition and destruction of tumor cell lines by human peripheral blood cells. The ability of human peripheral blood monocytes to control proliferation by transformed cells was studied in normal individuals and patients with malignancy. Effector mechanisms of this growth inhibitory activity and its regulation were distinct from those usually attributable to other immune cells or effector molecules. An enzyme-lined immunosorbent assay (ELISA) method to simply and rapidly detect the production of monoclonal antibodies directed against cell surface antigens was developed and implemented. This technique allowed selection of "hybridomas" producing antibodies specific for human monocyte components.